Variable displacement hydraulic pumps and motors are often used as a rugged, reliable and convenient way to transfer drive shaft power in a controlled manner. Such hydrostatic drives are used in construction vehicles and equipment, agricultural machinery, materials handling equipment, maritime vessels, machine tools, garden tractors and recreational vehicles.
In the usual application, a variable displacement pump is driven by a power source, such as a diesel or gasoline engine, turbine or electric motor. Flexible hydraulic lines or hoses connect the pump output to a hydraulic motor that drives the load.
In some instances in the past, pump displacement has been controlled by a manual lever throughout the range from zero to full flow in either direction. This provided an infinitely variable transmission ratio to the load, from full forward to full reverse, without the use of a clutch, mechanical gear box, or other functionally equivalent mechanism.
In such an arrangement, the hydraulic motor can be conveniently located at the load, while the pump is proximate the power source. The transmission ratio can be changed quickly by mere manipulation of the control lever, without damage to the pump or motor. Full load torque is available at stall, and optimum engine speed can be maintained at all times.
In one prior art form of hydrostatic drive, the pump was a variable displacement piston pump having a pivotal swashplate for determining the length of stroke of the pump piston. The angle of this swashplate was set by a lever manually controlled by the operator to displace a control valve for regulating flow to a control piston which positioned the angle of the swashplate and thereby regulated the desired load speed. Often a series of levers, push rods, bell cranks, or cables, were used to connect the operator's control lever to the pumps stroking mechanism.
In order to eliminate such mechanical interconnection between the operator and the hydrostatic pump, a simple potentiometer was located near the operator to selectively provide an electrical command signal for an electrohydraulic servovalve which was substituted for the former control valve. Thus, an electrohydraulic control mechanism replaced the manual lever and control valve and acted to position the swashplate in response to electrical commands. Feedback of swashplate position to the torque motor of the electrohydraulic servovalve was either mechanical as by a lever and spring operatively interposed between the swashplate and the movable armature of the torque motor, or electrical as by a potentiometer operatively interposed between the swashplate and the coils of the torque motor so that a negative feedback signal responsive to the position of the swashplate was generated to be algebraically summed with the electrical command.